Motor vehicles are required to comply with increasingly stringent limits on noise and emissions imposed by federal, state, and local regulatory bodies. Since published research has demonstrated noise and emissions from an internal combustion engine are influenced by the time history of the fuel flow rate through the injector spray holes, or injection rate shape, considerable effort has been expended to adjust and control the shape of this injection flow rate curve in response to the specific requirements of a particular engine application. Most hydraulic methods of regulating the flow rate at the injector involve either the use of a partial restriction or alternate flow path upstream from the nozzle spray holes to regulate the amount of fuel to reach the exit of the injector. The function of the alternate flow path in prior designs has typically been to divert a portion of the fuel to either an accumulator or the pump fuel supply system via a second external outlet located on the injector. A number of different approaches to implement these two methods have been taken.
Some injectors include two springs for biasing the needle valve toward its closed position. U.S. Pat. No. 4,938,193 to R. Raufeisen et al. entitled Fuel Injection Nozzle provides one example of this type. The two springs allow the injector to open in two stages. The needle valve opens a first distance under the influence of only one spring at a first pressure substantially lower than required to overcome the second spring preload. During this first stage of injection, the flow rate through the injector is throttled at the needle valve tip. Once the second stage opening pressure is reached, the needle valve moves to the maximum travel limit imposed by the needle lift adjusting screw to allow unrestricted flow to reach the injector spray holes. For many fuel systems the pump plunger motion and resulting rate of pressure rise in the system vary with engine speed. At idle and low engine speeds where the rate of pressure rise is low, sufficient time is available for the first stage operation to significantly influence the initial rate of fuel injection. As engine speed increases, the transition to second stage operation occurs more rapidly and lessens or eliminates the first stage regulation. Consequently, two spring systems typically provide rate-shaping at lower engine speeds, but not distinct pilot and main injections.
A further approach to regulate the flow rate through the use of throttling is outlined in U.S. Pat. No. 4,987,887 to W. Kelly entitled Fuel Injector Method and Apparatus. Two stage injector operation is obtained by metering fuel through a reduced radial clearance for a portion of needle valve travel before increasing the flow path area to provide unrestricted flow to the spray holes at the maximum limit of needle valve travel. With this type of flow path area to needle valve positional relationship, both rate regulation, and in some fuel systems utilizing a low initial rate of pressure increase, pilot injection, may be obtained with this type of injector in a design that can be manufactured at a relatively low cost.
An implementation scheme for diverting a portion of the fuel pump delivery is discussed in U.S. Pat. No. 5,647,536 to Yen et al entitled Injection Rate Shaping Nozzle Assembly for a Fuel Injector. Needle valve position is used to open and close flow rate limited spill paths within the injector to connect high pressure supply and low pressure drain circuits in the fuel system for a period of time during the injection. The flow rate of fuel entering the combustion chamber is claimed to change in a predetermined time varying manor as a result of this injector design.
Since power output, emission requirements, and economic constraints vary considerably with different engine applications, methods in addition to the above-discussed prior art are still required. One area of particular relevance is discussed in U.S. Pat. No. 6,526,939 to Reitz et al. entitled Diesel Engine Emissions Reduction by Multiple Injections Having Increasing Pressure. For this approach, electronic control of fast acting valves on common rail fuel systems is used to produce multiple injections for the reduction of particulate and NOx emissions. While the added expense and complexity associated with this type of fuel system may be justifiable for some engine applications, others may benefit from a different more simplistic and robust hydraulic control method to create either rate shaping, or rate shaping with pilot or multiple injections through the use of flow path closure within the injector.